The present invention is in the field of semi conductor manufacturing and more specifically relates to improvements in apparatus for handling semiconductor wafers as they are carried through a planairzing machine.
As supplied to the planarizing machine, the surface of a wafer may exhibit departures from flatness. Typically, 25 wafers are loaded into a cassette which is hand carried to a complex machine called a planarizer The purpose of the planarizer is to render the front surface of the wafer flat to within a fraction of a micron. After each wafer has been rendered flat, the planarizer inserts it into a cassette. When the cassette is full, it may be removed from the planarizer and transported to any of a number of other processing machines.
The present invention cannot be fully appreciated without an understanding of the operation of the planarizer Once the cassette containing typically 25 pre-processed wafers has been inserted into the planarizer, the operation of the planarizer in processing the wafers is completely autonomous. The operations are carried out under control of a computer that controls the application of power to various actuators in response to sensed inputs in a time-coordinated manner. In order for its lengthy program of operations to be carried out without mishap, it is important that means be provided to interrupt the accumulation of positional errors. It can also be appreciated that the wafers must be handled carefully to avoid scratching the processed surfaces which are so fragile that they are never again touched by human hands.
The present invention can best be understood by following a typical wafer as it is moved through the planarizer. The wafer is extracted from the cassette by a portion of the planarizer called a robot. The robot carries the wafer to a fixed location known as the load station. After the robot has deposited the wafer at the load station and has moved out of the way, a different part (called a spindle carrier) moves into position above the wafer and acquires it. The spindle carrier transports the wafer to a rotating polishing platen and rotates the wafer while holding it in contact with the rotating polishing platen until the wafer has been rendered flat. Thereafter, the spindle lifts the wafer from the polishing platen and transports it back to the load station where the wafer is rinsed. After the spindle carrier has deposited the wafer at the load station and has moved out of the way, the robot picks up the polished wafer and carries it to a cassette and deposits the wafer in the cassette. Throughout these operations, the wafer is maintained in a horizontal position with the side that is to be polished facing downward. That downward facing side of the wafer is also referred to as the front side of the wafer. The path of an individual wafer through the planarizer will be depicted in greater detail below.
Upon reflection it will be seen that the main purpose of the load station is to accurately center the wafer into the carrier notwithstanding the errors in their relative positions as they enter the load station.
In addition to centering the wafer with respect to the carrier, the load station performs a number of useful ancillary functions. The load station includes provision for sensing the presence of the wafer at the load station, for rinsing the wafer, for lifting the wafer into the carrier, and for supporting the processed wafer on cushions of water to avoid direct contact. Thus, the load station serves several other functions in addition to centering the wafer with respect to the carrier.
As the spindle carrier descends over the load station, it needs a way of determining whether it should descend to a first position suitable for depositing a wafer onto the load station or whether it should descend to a lower second position suitable for acquiring a wafer that is already present on the load station. It is an objective of the present invention to provide a way of making this determination
The load station is provided with three upwardly-directed nozzles for use in supporting a wafer on three separate spaced cushions of purified water or other suitable fluid. The nozzles are supplied through branch conduits from a supply main.
When no wafer is present, the pressurized water meets with little resistance as it is discharged from the nozzles. Accordingly, the pressure in the branches is relatively low.
In contrast, when a wafer is present, the wafer partially impedes the discharge of the water from the nozzles, causing the pressure in the branches to be greater than when no wafer is present.
In accordance with the present invention, the pressure in one of the branches is sensed. A low pressure indicates the absence of a wafer and signifies that the spindle carrier should descend to the first position to deposit a wafer. A high pressure indicates the presence of a wafer on the load station and signifies that the spindle carrier should descend to the lower second position to acquire the wafer.